My iPod 4 has a really weird battery

So I've had this iPod (4th generation) for about maybe 3 months now. It was my friends before mine. I have to keep it on the charger all the time, becuase it's battery is really messed up. It'll have a full battery after I've charged it, then 10 minutes later it'll be under 20% life left. Then, when it finally dies, I'm able to turn it back on and use it again. I'm not sure why it does this, any ideas on how to fix it?